Attaches one or more EC2 instances to the specified Auto Scaling group.

When you attach instances, Amazon EC2 Auto Scaling increases the desired
capacity of the group by the number of instances being attached. If the
number of instances being attached plus the desired capacity of the group
exceeds the maximum size of the group, the operation fails.

If there is a Classic Load Balancer attached to your Auto Scaling group, the
instances are also registered with the load balancer. If there are target
groups attached to your Auto Scaling group, the instances are also registered
with the target groups.